1. Field of the Invention
The present invention relates to a pilot operated hydraulic valves and more particularly to electrically operated pilot valves.
2. Description of the Related Art
A wide variety of machines have moveable components which are operated by an hydraulic actuator, such as a cylinder and piston arrangement, that is controlled by a hydraulic valve. Traditionally, the hydraulic valve was manually operated by the machine operator. There is a present trend away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a set of proportional pilot operated control valves arranged in a Wheatstone bridge to meter fluid flow into and out of the hydraulic cylinder, as shown in U.S. Pat. No. 6,149,124. Each control valve has a poppet that engages and disengages a valve seat to control the fluid flow between the inlet and outlet of the valve. A control chamber is formed on the remote side of the poppet from the valve seat and a control passage is provided between the inlet and the control chamber. The greater pressure from the inlet, that is applied to the control chamber, holds the poppet against the valve seat because of lower pressure at the outlet. An integral solenoid operated pilot valve is activated when it is desired to open the control valve by moving the poppet away from the valve seat. Pilot valve activation opens a pilot passage releasing pressure in the control chamber into the outlet, thereby allowing the poppet to move away from a valve seat. Closing the pilot valve and thus the pilot passage causes pressure in the control chamber to increase, forcing the poppet to close against the valve seat.
Therefore, a relatively small force, applied by the solenoid to the pilot valve, is able to operate a poppet on which a significantly greater force acts due to pressure from the supply pump or the hydraulic actuator.
The functionality of this type of electrohydraulic valve is limited to a relationship between the solenoid force and control chamber flow. While a smaller control chamber flow is still sufficient to position a substantially larger poppet, the speed of the poppet decreases rapidly with increases in poppet size. Another drawback is that the stroke of the poppet is incrementally less than the stroke of the solenoid. It is desirable that the solenoid provide both high force and a large stroke (to achieve large poppet strokes), thus the solenoid mechanism greatly constrains the scalability of this type of electrohydraulic valve.
One solution that has been attempted to achieve high flow and rapid response time placed a large variable orifice, that was proportional to the poppet stroke, in the control passage between the inlet and the control chamber. The pilot valve vented the control chamber pressure to the low pressure outlet port through either a balanced pilot or a spool valve. This valve design was relatively fast and proportional, but because of the high flows in the control chamber through the relatively large control passage orifice, additional pressure compensators were required to scale the valve to larger sizes. In addition, enlarging the control orifice resulted in a non-linear relationship between the fluid pressure and flow and the electric current applied to the solenoid, which non-linear relationship adversely affected accurate control.
Therefore, a need still exists for an electrohydraulic poppet valve that can rapidly control high flow rates that occur with large hydraulic actuators.